The present invention relates to a method of diagnosing a circuit for controlling a servomotor to check the circuit for any malfunctions, and more particularly to a method of diagnosing a servomotor control circuit for any malfunction in an LSI (Large-Scale Integration) circuit which constitutes a digital circuit in the servomotor control circuit.
It has been a general practice to employ a numerical control system for controlling a servomotor of a movable mechanism of a machine tool and the like in actuating the movable mechanism. The numerical control system has, in addition to a numerical control unit, a positional control circuit which mainly comprises a pulse distributor for carrying out arithmetic operations for pulse distribution based on positional or path data, a position detector such as a pulse coder for generating a positional pulse each time the servomotor rotates through a predetermined angular interval, an error register for computing and storing the difference (hereinafter referred to as an "error") between the number of distributed pulses produced by the pulse distributor and the number of positional pulses, a digital-to-analog converter (hereinafter referred to as a "D/A converter") for converting the error into an analog quantity, and a speed control circuit for driving the servomotor in response to an output from the D/A converter. The positional control circuit thus serves to drive the servomotor in response to the output from the D/A converter to position a tool or table in a commanded location or move the tool or table along a commanded path for performing desired machining on a workpiece mounted on the table.
The recent rapid progress of integrated logic circuit technology has reached the point where a great number of circuits can be incorporated on a single LSI chip. This advantage has allowed the pulse distributor, the error register, a portion of the D/A converter, and the digital control unit to be assembled as one LSI circuit, such that the positional control circuit is composed of a reduced number of circuit parts, is smaller in size, and is less costly to construct. The LSI circuit is also advantageous in that the circuit components assembled thereon are more reliable in operation. However, LSI circuits sometimes suffer from various failures such as a power supply fault, no clock pulse being supplied, and continued application of a clear signal. When such difficulties arise, the LSI circuit tends to be adversely affected to a large extent since it has a complex circuit arrangement due to an increased rate of integration. A malfunctioning LSI circuit causes a workpiece to be machined in error, with the results that the workpiece which is expensive, will be spoiled, the rate of machining the workpiece will be reduced, and mechanical parts of the machine tool will be damaged.